Due to the availability of high resolution scanners and low cost desktop printers, counterfeiting of security documents such as bank notes, checks, certification documents, identification cards, passports, travel documents, etc. has become a serious problem. The same is also true for valuable products such as DVDs, CDs, software packages, prescription drugs, cosmetics, fashion articles etc. that are often counterfeited.
The present invention proposes techniques for creating multi-ink color multi-halftone image prints by relying on two or more spatially laid-out halftone screens having different properties, such as different frequencies and different orientations. It includes methods for verifying the authenticity of these color image prints.
In several inventions, hidden watermarks are incorporated into halftones. In U.S. Pat. No. 7,020,349 “Halftone watermarking and related applications”, filed Mar. 28, 2006, to inventor Hugh L. Brunk, a watermark is incorporated into an output binary image by a modified halftoning process. The halftone screen is used to determine the geometric distortion of an image in order to facilitate the retrieval of the embedded digital watermark.
In U.S. Pat. No. 6,700,995 “Applying digital watermarks using dot gain correction”, filed Mar. 2, 2004, inventor Alastair M. Reed proposes to embed watermarks into a printed image by accounting for the dot gain of the printer. The color values of the watermark region of an image that is to be printed are modified according to the backward dot gain curves of the printer so as to leave the original signal value of the watermark. The teachings of that patent are limited to the modification of the color input image values.
There are prior art methods to create images incorporating security patterns that cannot be scanned without inducing strong artifacts by modulating the carrier screen (dot or line screen) by phase, frequency, angle, size, width or shape.
In Chapter 5, Sect. 4 of the book by Rudolf L. Van Renesse, Optical Document Security, Artech House, Third Edition, entitled Screen-Decoded images, pp. 146-169, methods are taught for incorporating a security message into a printed image, for example by dot phase modulation pp. 155-156. Locally shifted ink dots form a recognizable message. When a decoding dot screen overlay is placed on the printed ink dots, the hidden message is revealed. The presented method applies only to single ink pictures.
In U.S. Pat. No. 5,374,976, “Support provided with a machine detectable copying security element”, filed Dec. 20, 1994, inventors Sijbrand Spannenburg and Nt Petten teach a method for producing screen angle modulated single ink images (SAM). The SAM elements are composed of single ink fine line segments whose angular orientation is different on the inner and outer region of the encoded hidden message. In a scanned copy, due to the dot gain that depends on line segment orientation, the message hidden within the single ink printed image becomes visible. The presented method applies only to single ink pictures.
In contrast to the above mentioned disclosures, the present invention aims at creating multi-ink full color halftone images embedding a hidden message made of at least two spatially laid-out halftone screens whose frequencies, orientations or shapes differ one from another. When generating the parts of the image incorporating the hidden message, the effective ink surface coverages depend on the selected halftoning method. Halftoning methods of different frequencies, orientations or shapes exhibit different dot gain behaviors and therefore reveal the hidden message when reproduced by a potential counterfeiter. In order to hide the boundaries between different spatially laid-out halftones we present a method enabling finding the optimal boundary region connecting the differently halftoned parts. Thanks to the color prediction model based calculation of halftone dependent ink surface coverages and to the optimal boundaries connecting differently halftoned parts, the naked human eye is not able to distinguish between them. The message whose foreground and background are differently halftoned remains therefore hidden. The synthesized multi-ink multi-halftone images may represent continuous tone pictures of different origins such as photographs, graphic designs, paintings or synthetic images.